Cement is a
fine grey powder that, when mixed with water, sets to a hard mass. They possess
both adhesive and cohesive properties. They adhere to building units such as
bricks, stones, tiles, etc. because of which we are able to make walls and
floors of very high strength.
They are
considered hydraulic because they set and harden when mixed with water because
of the chemical combination of the cement compounds with the water, which
yields sub-microscopic crystals or a gel-like material with a high surface
area.
There are generally two types of
cement used on site. They are:
1. Ordinary Portland cement (OPC): This is the most commonly used cement. This type of cement
is manufactured in a powder form by mixing limestone and other raw materials
which consists of argillaceous, calcareous, and gypsum. It has adequate
resistance to dry shrinkage and cracking, but has less resistance to chemical
attack.
It is admirably suitable for use in the place where there is a need for
the fast construction and setting time, and where there is no exposure to
sulphates in the soil or in the ground.
OPC of different grades attains their maximum strength after
28 days.
OPC is generally used and widely preferred for the construction of
high-rise buildings, roads, dams, bridges, flyovers where high strength and
fast setting are required.
2. Portland pozzolana cement (PPC): It is an interground blended mixture of cement and
pozzolana. Pozzolana is a natural or artificial material containing silica and
alumina in a reactive form. These pozzolana materials include volcanic ash
burnt clay, fly ash, opaline shales, etc.
For the manufacture of PPC, about 25% of pozzolanic material is added to
the OPC clinkers and gypsums and the mix is thoroughly ground.
They possess higher tensile strength and evolve less heat during the
setting. They attain compressive strength with age, due to which they attain
less compressive strength in early days.
They are widely used for hydraulic structures such as dams,
sewage works, laying concrete underwater, etc. because they impart a higher
degree of water-tightness and offer higher resistance to chemical attack and
the action of seawater.
Generally,
we use OPC for constructing our houses and other structures. OPC cement is
available in the market in three grades. They are explained below:
1. OPC-33 grade:
For 33-grade cement, 33 means the compressive strength of cement should be 33
N/mm2 (Mpa) after 28 days. This grade of cement is not widely used and can only
be recommended for low-strength works like plastering.
2. OPC-43 grade:
For 43-grade cement, 43 means the compressive strength of cement should be 43
N/mm2 (Mpa) after 28 days. They are generally used in non-RCC works, wall
plastering, and pathways.
3. OPC-53 grade:
For 53-grade cement, 53 means the compressive strength of cement should be 53
N/mm2 (Mpa) after 28 days. This is a widely used grade of cement and is used in
the RCC structures, buildings, pre–stressed concreting, heavy structures, etc.
Quality check of cement
on site
1. Manufacturing Date of cement or Packaging Date: There might not be physical changes
in cement with time but the strength of cement reduces with age. This is
because cement is in the form of powder and it has the property of absorbing
moisture from the air quickly. When the cement bag absorbs the moisture from
the air, the initial setting takes place and the cement will start to lose its
strength gradually.
The cement should have to be used within the 3 months from
its manufacturing date. This is because the cement used after 3 months of
manufacturing date loses its strength by approximately 20-30%. They lose their
strength by 30-40% after 6 months and by 40-50% after 12 months from the
manufacturing or production date.
2.
Colour test:
The cement should have a uniform grey color with a light greenish shade.
Cement color indicates the presence of excessive clay or lime.
3.
Lumps Test:
The sample of the cement must be free from the presence of lumps that are
formed due to the absorption of moisture from the atmosphere by the cement.
This process is termed hydration.
By inspection and supervision, we can conclude the presence of lumps and
the cement bag with lumps should be rejected.
4.
Float Test:
Take a hand full of cement and throw it in water, if the cement floats
on the surface of the water for some time before sinking, then it is a good
quality cement. If the cement sinks immediately, then it is bad quality of
cement. Also, if the cement floats on the surface of the water without sinking,
then the cement is of bad quality.
5.
Temperature Test:
Cement tends to remain cool when it is in a bag. When we touch the cement
from the bag, we feel the presence of cool (cold) in our hands.
If the cement is cool inside
the bag, then the cement is of good quality. If the cement is warm inside the
bag, then the process of hydration has already taken place and cement is of bad
quality.
6.
Rubbing Test for adulteration:
Adulteration is the process of making something poorer in quality by the
addition of another substance.
Take a pinch of cement and rub it
within your fingers. If it gives a smooth feeling, then the cement is of good
quality.
If it gives a Gritty or rough feeling, then there is an adulteration
of silt or sand in the cement and is of bad quality.
7.
Setting Test:
Take about 100gm of cement and a small quantity of water and make a stiff
paste of it. Then prepare a cement cake with sharp edges and put it on a glass
plate and slowly take it underwater in a bucket.
Observing after 24 hours, the cake should remain in its original shape
and it should attain some strength, for the cement to be of good quality, as
shown in the figure below. The figure shown below is of good quality cement
since there are edges.
8.
Strength Test:
Prepare a block of 25mm*25mm*200mm from the given sample of cement and
immerse the block in water for curing for 7 days.
Remove the block of cement from
water and place it over the support that is 150mm apart and subject the block
to a load of 340N (or 34kg weight). If the block does not show any failure,
then the cement is of good quality.
